1. Field of the Invention
The present invention relates to an apparatus for adjusting the channel width of a multi-channel fiber amplifier light source, and more particularly to an apparatus for adjusting the channel width of a multi-channel fiber amplifier light source for wavelength division multiplexing (WDM), including a Fabry-Perot filter rotatably disposed in a fiber-optic beam expander.
2. Description of the Prior Art
Erbium-doped fibers are generally made by doping erbium that is a kind of rare-earth element in conventional optical fibers made of silica glass.
When signalling beams pass through an erbium-doped fiber together with exciting or pumping beams, the erbium-doped fiber converts energy of the exciting beams with the wavelength of 980 or 1,480 nm into signalling beam energy, thereby amplifying the optical signal.
When only the signalling beams pass through the erbium-doped fiber under a condition that no exciting beam is applied, the erbium is excited by the signalling beams, thereby spontaneously emitting beams. These beams spontaneously emitted is then amplified while travelling along the fiber. Due to such an amplified spontaneous emission (ASE), only noise beams are generated over the wavelength range from 1,530 nm to 1,560 nm and then outputted through the output terminal of the erbium-doped fiber.
Although such noise beams were handled as an unnecessary factor to be removed, there has recently been made a proposal for using them as a broadband WDM light source, namely, a fiber amplifier light source by passing them through an optical filter or optical modulator. This proposal was made by AT & T, U.S.A. in 1993. This method provides an advantage of substituting a plurality of WDM light sources by a single fiber-optic amplifier. It is also possible to easily set the center frequency of each channel meeting the standards by use of an optical filter.
Generally, beams emitted from the above-mentioned light source are irregular in terms of wavelength and phase, as different from those outputted from existing optical communication laser diodes. They have a characteristic similar to that of irregular thermal radiation rays. Such a characteristic results in a poor quality of the light source. In this regard, an improvement in receiving sensitivity may be achieved by increasing the line width of channels as compared to the transfer rate.
However, an excessively increased channel line width results in a distortion of optical pulses due to the color dispersion effect of optical fibers. As a result, there is a restriction on transfer distance.
Furthermore, when ASE components of beams are removed using the optical filter, the efficiency of the exciting beams effecting the power of the signalling beams to be finally used is correspondingly reduced.